1. Field of the Invention
The present invention relates generally to optical devices, and more particularly to an optical sensor which is specifically configured to be utilized in conjunction with an image processing technique in order to provide a much higher level of resolution without significantly increasing the size of the sensor.
2. Description of the Prior Art
Optical sensors are devices which for decades have been utilized to detect and record various optical images. Various types of optical sensors have been developed which work in the the Ultra Violet Bands, Infra Red Bands as well as in the Visible Bands of operation. Examples of such devices include Weather Sensors, Terrain Mapping Sensors, Surveillance Sensors, Medical Probes, Telescopes and Television Cameras.
An optical sensor typically includes an optical system and one or more detectors. The optical system portion is made up of various combinations of lenses, mirrors and filters which is utilized to focus light onto a focal plane located at the image plane of the optical system. The detectors which make up the image plane is utilized to convert the light received from the optical system into electrical signals. Instead of detectors, other types of optical sensors utilize film to record the images. In this case, the grain size of the film is analogous to the detectors described above.
An important performance characteristic of optical sensors is known as "spacial resolution" which is the size of the smallest object that can be resolved in the image or equivalently is the ability to differentiate between closely spaced objects. If the optical system of these sensors are free from optical aberrations which means being "well corrected" the spacial resolution is ultimately limited by one of two factors. Either the resolution is limited by the size of the detectors in the focal plane or by diffraction effects.
Diffraction is a well known characteristic of light which among other things describes how light passes through an aperture of an optical system. Diffraction causes the light passing through an aperture to spread out which causes points of an image not to be a point, but rather a pattern of light known as a diffraction pattern diffused across a focal plane. For a well corrected unobscured optical system known as a diffraction limited system, the diffraction pattern includes a very bright central spot, surrounded by bright and dark rings of much less intensity which decreases in intensity as the distance from the ring to the central spot increases.
An optical sensor that is designed to be diffraction limited, typically, has a very well corrected optical system and detectors sized so that the central spot of the diffraction pattern just fits within the active area of the detector. Making the detectors any smaller does not serve any purpose and is considerably more costly due to the added cost of the extra detectors and the associated electronics
The size of the aperture utilized in the optical system also is an important factor due to diffraction effects. The size of the aperture is expressed differently depending on the type of application. In applications such as camera lenses and telescope objectives, the aperture is normally expressed as a f-number which is the ratio of the effective focal length to the size of the clear aperture. In applications such as microscope objectives, the aperture is normally expressed as a Numerical aperture (NA) which is the index of refraction times the sine of the half angle of the cone of illumination. For a given focal length, a high f-number corresponds to a smaller aperture, while a higher Numerical aperture corresponds to a larger aperture.
A drawback with conventional optical sensors relates to the size of the aperture required for a given level of resolution. In order to increase the resolution of such devices, a larger aperture is required. In many situations, the use of such a system is very costly. This is because utilizing a larger aperture requires the use of a significantly larger optical system. The cost for larger systems which have apertures with diameters greater than one foot is proportional to the diameter of the aperture raised to a power of "x". The variable "x" typically ranges from 2.1 to 2.9 depending on a number of other particulars associated with the sensor such as its wave band, field of regard and field of view.
The consideration for the size of the optical sensor is particular relevant in systems that fly on some type of platform, either in space or in the air. Under such conditions, the sensor must be light weight, strong and capable of surviving the rigors of the flight environment. Thus, the cost of going to a larger optical system can be in the hundreds of millions of dollars for some of the larger and more sophisticated sensors. Further, the size of the sensor may also be limited by such practical considerations as the amount of weight or space the host rocket, plane, balloon or vehicle accommodates. In these situations, a larger system cannot be implemented regardless of the cost.
A number of optical imaging techniques have been developed which are directed at increasing spatial resolution. One such example attempts to increase the resolution of optical sensors by utilizing a condition known as sub-pixel resolution. In sub-pixel resolution, the optical system is limited in spacial resolution not by diffraction, but by the size of the detectors or pixels. In this case, a larger detector size is utilized to prevent a portion of the higher spatial frequencies of the image formed by the optical system from being observed. Thus, sub-pixel resolution attempts to reconstruct an image that includes these higher spatial frequencies which are already in the image. This technique does not attempt to reconstruct an image that is smaller than the diffraction limit, which is even smaller than the sub-pixel resolution. Further, this technique also does not require hardware or system operation changes in order to achieve sub-pixel reconstruction. Examples of sub-pixel resolution techniques are disclosed in an article ADVANCED SIGNAL PROCESSING ALGORITHMS, ARCHITECTURES AND IMPLEMENTATIONS II, by J. B. Abbiss et al., The International Society For Optical Engineering, Volume 1566, P. 365.
Other examples of optical imaging techniques are disclosed in another article entitled SUPERRRESOLUTION ALGORITHMS FOR A MODIFIED HOPFIELD NEURAL NETWORK, by J. B. Abbiss, IEEE Transactions On Signal Processing, Vol. 39, No. 7, July 1991 and in a paper entitled FAST REGULATED DECONVOLUTION IN OPTICS AND RADARS, by J. B. Abbiss, presented at the 3rd IMA Conference on Mathematics in Signal Processing. These techniques utilize linear equation and matrix techniques in order to restore signals or images from a limited discrete data set.
The previously described techniques have a number of drawbacks in regard to optical sensors. First of all, only one of these techniques is directed toward a diffraction limited device. Also, these techniques often produce forms of solutions which are not practically solved due to the constraints of computing power in airborne systems. This is because such solutions often yield large rounding errors and require a large number of operations. Further, none of the previously described techniques disclose either the types of detectors or other system parameters which are utilized along with these techniques.
It is therefore, an object of the present invention to provide a technique for optimally increasing the resolution of an optical sensor that is diffraction limited without utilizing a substantially larger aperture.